Transpiration is the act of excreting a liquid, vapor, or gas through a surface as a means of cooling the surface. This type of thermal protection is particularly desirable in a high heat environment such as is experienced by nose cones for re-entry vehicles, missile leading edges and nose tips, rocket nozzles and combustion chamber linings, steam and gas turbine blades, and instruments exposed to a high heat flux.
As an aircraft body exceeds supersonic speed it approaches a complex atmospheric condition wherein considerable heat conditions are imposed upon the craft sufficient to rapidly melt, and/or vaporize most known materials at an extremely rapid rate similar to the burning and destruction of meteors. This may occur in atmospheric flight, leaving or re-entering the atmosphere. Missiles re-entering the atmosphere at very high velocity are decelerated by pressure and frictional resistance and the nose portion of such missiles is subjected to intense heating.
One solution for protecting missiles during re-entry is the use of a heat shield which insulates the forward portion of the vehicle from severe heating by partial ablation, re-radiation, and absorption. Another approach is to circulate a coolant through passageways adjacent the most severely heated portions of the craft and then ejecting this coolant from the vehicle through porous, external elements for further cooling the heated portions. Coolant thus ejected by transpiration through the porous material provides cooling during evaporation of the coolant and by the formation of a boundary of vaporized coolant for shielding the external surfaces from adjacent, superheated, gaseous flow.
In the past coolant has been stored in a container in the vehicle and pumped, when needed, to the heated, external surface of the vehicle. Such control systems, pumps, and the like, add undesirable complexity and weight to the cooling system and are subject to possible malfunction or failure under stress.